Narrow band interferences also referred to as Ingress are often present in the return path of cable systems. Ingress noise enters the cable system through defective shields, and couples with the signal. It consists of narrow band signals with power as high as +10 dB over the carrier power and bandwidth typically less than 20 KHz. Sources of ingress include ham radio transmissions and therefore can be intermittent. Ingress may slowly drifts in frequency. Channel impairments also include channel amplitude and phase distortions, burst/impulse noises, and thermal noises.
Cable Modems (CMs) use the return path to transmit short signals, referred to as bursts, to the Cable Modem Termination System (CMTS) located at the head end. The Media Access Control (MAC) layer of the CMTS uses Time Division Multiple Access (TDMA) or Synchronous Code Division Multiple Access (S-CDMA) to control the access to the return path by the CMs, and avoid burst collisions. Transmissions occur inside time slots. The CMs must start transmitting their bursts early enough so they reach the CMTS at the time slots that were allocated to them. The CMTS synchronizes to the incoming bursts prior to extracting the data. Synchronization is facilitated by means of a preamble. The preamble represents a sequence of symbols that is known to the CMTS.
To facilitate inter-operability between CMTS and CMs, a Standard known as Data Over Cable Service Interface Specification (DOCSIS) and which is employed by many cable operators, was developed to define the communication protocols between CMTS and CMs. In DOCSIS, two modes of operations are used in the return path: ranging and traffic modes. Ranging is used when a CM joins the network and also for periodic maintenance. The process of ranging is similar for TDMA and S-CDMA modes, except that timing synchronization requires higher accuracy in S-CDMA. If TDMA is selected, a CM which has ranged successfully enters the traffic mode to transmit TDMA bursts. Similarly, if S-CDMA is selected, a CM which has ranged successfully enters the traffic mode to transmit S-CDMA bursts.
Different preambles are used in ranging and traffic modes. In ranging mode the preamble is inserted at the beginning of the burst, and includes a sync sequence and a training sequence. The sync sequence is used by the CMTS to detect the start of the burst, recover timing, measure the signal level, and correct for phase and frequency offsets. The training sequence is used for equalizing the channel. During the ranging mode, the CMTS processes the received burst transmitted by the CM to both measure the CM synchronization parameters (i.e., power level, carrier frequency and timing offsets), and compute the coefficients of the CM pre-equalizer. Following processing of the burst, the CMTS transmits the information back to the CM using the downstream channels, so the CM can adjust its synchronization parameters and configure its pre-equalizer before transmitting a new burst in the return path (i.e., upstream channels). Pre-equalization at the CM allows to cancel the effect of the channel at the CMTS. The CMTS informs the CM to operate in traffic mode when the received burst is determined to be of sufficient quality for communicating in TDMA or S-CDMA, whichever modes has been selected. CMs periodically return to ranging for adjustments of the pre-equalizer coefficients and/or synchronization parameters.
In TDMA traffic mode, the preamble is inserted at the beginning of the burst, and consists of a sync sequence, that is used to estimate residual timing offset, phase offset and signal level. In S-CDMA traffic mode, the preamble is interleaved with the data, and is used to estimate phase offset and signal level.
Since ingress does not depend on which CM transmits, it is natural to suppress the narrow band interferences (i.e., ingress) at the CMTS, so in the receiver. There are several methods to suppress the narrow band interferences. One method uses a prediction filter to extract the interferences so they can be subtracted from the communication signal. A prediction filter is an adaptive FIR notch filter whose coefficients can be adjusted using a gradient algorithm such as for example the Least Mean Square (LMS) algorithm. The prediction filter coefficients are usually adjusted when no CM transmits so only ingress noise and broadband noises are present in the channel, or in other words, there is no communication signal. The coefficients are kept unchanged when the CMs transmit. There is provision in the DOCSIS Standard to reserve time slots when no CM transmits to allow adjustments of the prediction filter coefficients.
The suppression of interferences using a prediction filter when the communication signal is present causes distortions to the communication signal. A Decision Feedback Equalizer (DFE) can be used to compensate for the distortions introduced by the prediction filter.
An alternative technique to suppress ingress noise is to use an Infinite Impulse Response (IIR) notch filter instead of an FIR notch filter, IIR notch filters can filter interferences with significantly less coefficients than their FIR counterparts.
IIR notch filters can cause severe distortions to the communication signal, while suppressing the interferences. As in the case of an FIR filter, a DFE can be used to compensate for the signal distortions introduced by the notch filter, as described in the paper by G. Redaelli, et al.:“Advanced Receiver to Dip Ingress Noise in HFC Return Channel”, ISPACS 2000 conference proceedings.
The cascade of an IIR/FIR notch filter and a DFE is an effective solution to suppress narrow band interferences but has two main problems. Problem 1: Synchronization to the burst signal is required for the DFE to properly operate, however high-power interferences mask the burst signal and must be notched in order to synchronize. Notching the interferences causes severe distortions to the burst, which adds to the distortions caused by the channel. This plurality of distortions renders synchronization very complex to achieve. Problem 2: The DFE simultaneously compensates for both channel distortions and distortions caused by the notch filter. This is expensive to implement as different DFE coefficients are needed for each CM, since channel distortions depend on which CM transmits. Also, the DFE coefficients must be re-computed for each CM every time the interferences change, which adds significant overhead to the system.
An effective approach to solve problem 1 is to “build” a notch filter that minimizes distortions to the communication signal in order to synchronize. Distortions introduced by an IIR notch filter are mostly phase distortions. A method was proposed to build a zero phase shift IIR notch filter (i.e., a filter which only causes amplitude distortions to the signal) in the paper by J. J. Kormylo, et al., “Two-Pass Recursive Digital Filter with Zero Phase Shift”, IEEE transactions on acoustics; speech, and signal processing, October 1974. As described in Kormylo's paper, a zero phase shift IIR notch filter can be built from a conventional IIR notch filter by performing a two-pass filtering of the received burst signal through the conventional IIR notch filter, the first pass occurring in the forward direction as the burst signal is received and the second pass in the time reverse direction after receiving the full burst and storing the notch filter output in a memory so it can be read backward from the memory. The output of the second-pass filtering is also stored in memory, since it has to be read backward again for processing the burst. This method is quite expensive and time consuming since it requires two times buffering of the complete burst before the burst can be processed to recover the data.
An effective approach to solve problem 2 is to isolate/decouple channel equalization from the suppression of interferences. A solution was proposed in the case of a prediction filter in cascade with a DFE in U.S. Pat. No. 7,843,847 (Quigley) issued November 2010 to Broadcom. The coefficients of the prediction filter are adjusted when no CM transmits so only when narrow band interferences are present in the channel. A DFE comprises a feedforward equalizer and a feedback equalizer. The feedback equalizer is used to compensate for the distortions introduced by the prediction filter. This is achieved by setting the feedback equalizer coefficients equal to those of the prediction filter coefficients. The feedforward equalizer is solely used to compensate for the channel distortions. The coefficients of the feedforward equalizer are adjusted during the ranging mode, while feedback equalizer and prediction filter coefficients are kept unchanged. This mechanism allows to decouple channel equalization from ingress noise removal in the case of a FIR notch filter in cascade with a DFE. The coefficients of the feedforward equalizer are sent to the CM for configuration of its pre-equalizer.
The following references are relevant to this field:
U.S. Pat. No. 7,843,847 (Quigley) issued November 2010 to Broadcom Corporation discloses a number of features for enhancing the performance of a communication system, in which data is transmitted between a base station and a plurality of subscriber stations located different distances from the base station. The power transmission level, slot timing, and equalization of the subscriber stations are set by a ranging process. Data is transmitted by the subscriber stations in fragmented form. Various measures are taken to make transmission from the subscriber stations robust. The uplink data transmission is controlled to permit multiple accesses from the subscriber stations
U.S. Pat. No. 7,826,569 (Popper) issued May 2010 to Juniper Networks, Inc. discloses a device for reducing ingress noise in a digital signal, comprising a noise predictor for predicting an amount of ingress noise in the digital signal based on past samples of the ingress noise, and a subtractor for subtracting the predicted amount of ingress noise from the digital signal. Channel distortion is compensated for by a noise-independent equalizer, such as a ZF equalizer, placed upstream of the noise predictor. The device may be incorporated, for example, in a cable modem termination system (CMTS) of an hybrid fiber/coax (HFC) network.
U.S. Pat. No. 7,716,712 (Booth) issued May 2010 to General Instrument Corporation discloses an adaptive data stream filter which removes narrowband interference from the CATV return path prior to these paths being combined in the network. This provides a method for removing narrowband interference from the CATV return path which detects potential narrowband interference in real-time and adapts a filter to remove this potential narrowband interference. The method uses previously created filters that are combined based on detected interference in an adaptive manner to continually adapt to new interference sources. Another embodiment calculates new filter coefficients for the data stream filter based on detected interference. In another embodiment, two filters are operated in a ping-pong manner for each band of interference identified as above threshold. This enables updating of one filter while another filter is performing the data stream filter operation
U.S. Pat. No. 6,360,369 (Mahoney) issued March 2002 to Broadcom Corporation discloses a system for the removing of interference (ingress) in cable modems without reducing the data rate or changing the frequency of the signal. A variable band stop filter bank and a non-linear equalization system are used. The band stop filter bank removes ingress while the equalization system, comprising an inner and an outer equalizer, removes the distortion created by the band stop filtering. A spectrum monitor detects both the presence of ingress and its frequency, and then feeds the data to a digital signal processor which calculates the distortion removal settings for the equalizers.
U.S. Pat. No. 6,285,718 (Reuven) issued September 2001 to Orckit Communication Ltd discloses an apparatus for transmission of high speed data over communication channels including a modulator which modulates an outgoing stream of digital data to generate an outgoing signal, and a demodulator which demodulates an incoming signal to generate an incoming stream of digital data, wherein the modulator comprises a band suppressor for suppressing portions of the outgoing signal which have specified frequencies.
G. Redaelli, et al., “Advanced receiver to dip ingress noise in HFC return channel”, ISPACS 2000 conference proceedings, 2000
J. J. Kormylo, et al., “Two-pass recursive digital filter with zero phase shift”, IEEE transactions on acoustics, speech, and signal processing, October 1974
M. D. Macleod, “Fast nearly ML estimation of the parameters of real or complex single tones or resolved multiple tones”, IEEE Transactions on Signal Processing, January 1998
A. V. Oppenheim, and, R. W. Schafer, “Discrete-time signal processing”, Prentice-Hall, 1st ed, 1989, chap, 10
The disclosure of each of the above documents is hereby incorporated by reference.